Lanthanide (Er, Eu)-Doped TiO₂ with Enhanced Kinetics for a High-Performance Asymmetric Supercapacitor and Oxygen Evolution Reaction

Supercapacitor performance is generally hindered by sluggish electrode reaction kinetics, which can be improved through doping to create surface defects, oxygen vacancies, and active sites. Pristine TiO2, Er-doped, Eu-doped, and Er/Eu-codoped TiO2 nanoparticles were synthesized via a solution-based method. Among all of the electrodes fabricated, Er/Eu-codoped TiO2 showed the highest specific capacitance of 1772.5 F g−1 at 1 A g−1 and a low charge transfer resistance of 12.37 Ω, highlighting the enhanced electrical conductivity and rapid ion and electron transport. The Er/Eu-codoped TiO2 electrode was further investigated in asymmetric supercapacitor geometry, where it demonstrated a wide potential window of 1.6 V, a specific capacitance of 135 F g−1 at 2 A g−1, an energy density of 48 Wh kg−1, and a power density of 1.6 kW kg−1. Additionally, pristine TiO2, Er-doped, Eu-doped, and Er/Eu-codoped TiO2 exhibited excellent oxygen evolution reaction performance, with onset potentials of 1.57, 1.52, 1.49, and 1.47 V and low overpotentials of 460, 400, 380, and 290 mV at 20 mA cm−2. The electrodes exhibited excellent electrochemical stability when tested for 20 h using constant potential electrolysis. Tafel slopes of 166.11, 164.71, 139.08, and 115.19 mV dec−1 suggested that all of the electrocatalysts under study are promising materials for OER research. These findings confirm that TiO2 codoped with rare earth metals is an appealing electrode material for energy storage as well as energy generation.